OLED devices, i.e. organic LED devices are area light sources in contrast to normal LEDs, which can be generally described as point light sources rather than area light sources. One feature which can be realized using OLED technologies is to have an area light source which is transparent in the off-state.
The state of the art concepts for transparent or partially transparent OLEDs are based on the use of thin, low absorbing cathodes. In other words, the cathode is made of transparent or translucent material. As the anode and also the organic layer of the OLED device are transparent as well, the complete OLED device is generally transparent. In this context it may generally be noted that the term “transparent” is used throughout this application also as a synonym for “translucent”. “Essentially transparent” is generally defined to be a state of a particular layer or device in which light within the visible frequency range can pass through with a loss of less than 80%, preferably 50%, most preferred 20%. Thus, “essentially opaque”, which is the opposite of “essentially transparent”, is always reached when light is absorbed at a ratio of at least 80%. “Layer” throughout this description can comprise a single integral layer, but also a stack of layers, in particular with reference to the organic layer.
For instance, US 2011/0193477 A1 discloses such an OLED device with an organic light-emitting layer between a transparent anode and a transparent cathode. In addition, at that side of the cathode facing away from the organic layer a structured mirror layer is disposed.
In order to realize a transparent cathode, often silver-based cathodes are used. One disadvantage connected with such thin silver layers is that they suffer from reliability problems. In addition, silver is a costly and comparatively rare material. Other materials such as so-called TCOs, i.e. transparent conductive oxides such as ITO (indium tin oxide), can also be used theoretically, however, the organic layer which emits the light can easily be damaged during the deposition and/or curing of such layers.
Therefore, it is an object of the invention to provide an alternative solution of how to realize an OLED device which is transparent for at least some ambient light to pass through and easy to both produce and to maintain at the same time.